The present invention relates to a method of producing blow-molded synthetic resin articles utilizing a blow-molding machine which produces at least one preform with a wall varying along its length utilizing a nozzle head with a nozzle gap width controlled by a computer unit based upon comparison of actual values with stored values and under program control.
More particularly, this invention relates to a method of producing blow-molded articles in this manner with improved computer control of the products which are produced.
A blow-molding machine for synthetic resins having a nozzle head through which the parison or preform is extruded and having its nozzle gap width controlled by a computer unit is described in DE 36 19 113 A1. This blow-molding machine for synthetic resins has a double-conical head for forming a cylindrical body which is extruded from a nozzle whose nozzle gap width is determined between the lower end of a mandrel and the inner lower end of a double-conical head by the vertical movement of the mandrel which adjusts or sets the gap.
The injection stroke of an injection piston is provided with a sensor connected to the setting unit which allows the setting of the thickness profile of the nozzle slit. The setting device is also associated with a setting value for a setting cylinder and a marking device with the aid of which markings can be applied to the extruded preform at certain distances in the longitudinal direction or direction of extrusion thereof. The adjusting system can further include a thickness-measuring unit which can determine the cross section or wall thickness of the shaped article, i.e. the finished article.
The determination of the wall thickness control curve and its use in adjusting the nozzle gap which is a programmed function of that wall thickness control curve utilizes the markings at predetermined spacings in the longitudinal direction for a comparison with the measured cross section or the measured wall thickness to ensure that the control unit will establish a predetermined thickness at each critical point in the length of the article which is produced by suitably controlling the thickness of the extruded parison at the region determined by the marking.
The nozzle gap is thus adjusted by control scanners from the control unit, namely the computer based upon a comparison with the actual gap width and the programmed gap width as determined at the marked point by the wall thickness control curve. As a consequence a length signal is obtained which is a function of the determined length of the preform and a piston position signal is obtained from the sensor while a cross section or wall thickness signal is obtained form the thickness-measuring device and these signals are so supplied to the control unit, namely, the computer that a constant wall thickness and the value of the thickness cross section as a function of the parison length is displayed visually on a display screen of the computer and is maintained automatically.
From the German Journal xe2x80x9cKunststoffexe2x80x9d (Synthetic Resins) 74 (1984) (1984), Volume 1, pages 9-11, wall thickness optimization in a blow molding is known utilizing microcomputer control based upon partial wall thickness control systems (PWDS). The microcomputer which is used has an integrated wall thickness program (firmware) which controls the wall thickness of the preform or parison in its length and peripheral directions in addition to control of the temperature of the extrusion cylinder and the temperature of the extruded mass utilizing a cascade control, and also controls the movements of the blowing mandrel and the transport and mold closure systems.
By storage of predetermined setpoint values in an external data storage or memory of the computer, apart from the points in the profile of the article, all other data which determine the wall thickness distribution over the length of the article are stored and can be called up for producing the particular article. The program in the computer can rely on such data. The markings on the parison or preform which result in the particular wall thickness profile and cross section can be displayed for the operator and designer on the display unit of the microcomputer.
It is the principal object of the present invention to improve the method of making blow-molded synthetic resin articles so as to simplify that method, improve the quality and reproducibility of the articles made and ensure greater precision in the actual molding of the articles based upon a wall thickness control curve.
Another object of the invention is to provide a method of making such articles whereby drawbacks of earlier systems are avoided.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention, in a method of producing blow-molded articles from synthetic resin which comprises the steps of:
(a) blow molding an elongated hollow article with a wall thickness varying along a length thereof on a blow molding machine having a computer-controlled blow molding nozzle with a nozzle gap width varying during the blow molding of the article in accordance with a stored program and comparison of measured values of the gap width with programmed values thereof;
(b) displaying an image of an article blow molded in step (a) on a screen of a computer controlling the blow molding machine alongside an outline of a wall thickness control curve generated by the program and represented an intended wall thickness of blow molded articles to be produced; and
(c) marking critical locations of the wall thickness control curve on the displayed image.
More particularly, the finished article, usually in the form of a photograph thereof, is scanned and an image of the finished article is displayed on a screen of the computer unit alongside the profile of the wall thickness control curve and critical locations of the curve profile are marked and graphically associated with the displayed image. This allows modification of the curve profile at the critical marked points by the control unit to accommodate deviations of the actually produced article from the desired shapes or profiles or wall thicknesses at these locations.
The operator can, without delay, recognize a deviation of the shape or wall thickness at the critical locations by a visual comparison on the screen and bring about immediate correction. The photograph may be a photograph obtained by conventional film photography and scanned utilizing a scanner, or a photograph procured by an electronic scanner and transferred electronically to the computer.
The viewer can immediately see when a critical location, marked on the screen in the manner described, does not correspond to the documented finished article so that some adjustment of the nozzle gap can be effected. The process can be repeated until the shape of the finished article and the wall thickness distribution thereof is at optimum.
The scanning in of photographs or otherwise providing the computer with true to nature images of different finished articles allows the programs for operating the blow-molding machine and introducing the finished articles to be tweaked or tuned to the actual products made so that it is no longer necessary to store large numbers of finished articles for documentation of the program or to provide wall thickness comparisons or setpoint values for the respective processes for producing the particular articles.
In the simplest case, the variable nozzle gap at the output side of the blowing head for the parison or preform is used for control. To avoid variations in the weight of the preform, the stroke of the mandrel is controlled based upon a factor which includes a basic component and a profile component in addition to a linear component, depending upon the length of the parison and the material used.
Furthermore, the profiles of the wall thickness curves of different finished articles and/or varied profile curves can be stored in the computer or in an associated memory and the operator can access an archiving system which allows program change by the selection of particular pages from the archive for display of the respective finished articles on the screen. In other words, not only is the data for producing a particular article stored, but the images of the finished articles associated with those programs are archived as well and can be called up as required.
As has been noted, deviations of the documented finished article actually produced from the wall thickness control curve can be readily determined and the control curve altered to ensure production of the desired finished article shape and wall thickness distribution. The different profiles of the wall thickness control curve, i.e. the original form and the modified forms associated with the archived finished articles and other curves which are to be imparted to various articles all can be stored and called up as required to modify the control curve or the process for producing a particular article. Each finished article produced by the machine as well as all of the data for producing can be archived together in a data base of the computer or separate but accessible memory.